CHAP. VII. SLEEP OF LEAVES. 339 



by 8.18 A.M., after the leaves had been illuminated for 2 h. 3 m., 

 and had acquired their diurnal position, they were placed in a 

 dark cupboard. They were looked at twice during the day and 

 thrice in the evening, the last time at 10 30 P.M., and not one had 

 become vertical. At 8 A.M. on the following morning (6th) they 

 still retained the same diurnal position, and were now replaced 

 before the north-east window. At night all the leaves which 

 had faced the light had their petioles curved and their blades 

 vertical ; whereas none of the leaves on the back of the plants, 

 although they had been moderately illuminated by the diffused 

 light of the room, were vertical. They were now at night placed 

 in the same dark cupboard; at 9 A.M. on the next morning (7th) 

 all those which had been asleep had reassumed their diurnal 

 position. The pot was then placed for 3 h. in the sunshine, so 

 as to stimulate the plants ; at noon they were placed before the 

 same north-east window, and at night the leaves slept in the 

 usual manner and awoke on the following morning. At noon on 

 this day (8th) the plants, after having been left before the north- 

 east window for 5 h. 45 m. and thus illuminated (though not 

 brightly, as the sky was cloudy during the whole time), were 

 replaced in the dark cupboard, and at 3 P. M. the position of the 

 leaves was very little, if at all, altered, so that they are not 

 quickly affected by darkness; but by 10.15 P.M. all the leaves 

 which had faced the north-east sky during the 5h. 45m. of 

 illumination stood vertical, whereas those on the back of the 

 plant retained their diurnal position. On the following morning 

 (9th) the leaves awoke as on the two former occasions in the dark, 

 and they were kept in the dark during the whole day ; at night 

 a very few of thorn became vertical, and this was the one in- 

 stance in which we observed any inherited tendency or habit in 

 this plant to sleep at the proper time. That it was real sleep 

 was shown by these same leaves reassuming their diurnal posi- 

 tion on the following morning (10th) whilst still kept in the 

 dark. 



The pot was then (9.45 A.M. 10th) replaced, after having been 

 kept for 36 h. in darkness, before the north-east window ; and at 

 night the blades of all the leaves (excepting a few on the back of 

 the plants) became conspicuously vertical. 



At 6.45 A.M. (llth) after the plants had been illuminated on the 

 same side as before during only 25m., the pot was turned round, 

 so that the leaves which had faced the light now faced the 

 interior of the room, and not one of these went to sleep at night; 



